1. Field of the Invention
This invention relates generally to information signal recording and reproducing apparatus and, more particularly, is directed to a gain control circuit for reducing the noise generally accompanying a reproduced information signal in an information signal recording and reproducing apparatus.
2. Description of the Prior Art
Noise reduction systems for reducing noise and distortion which accompany a reproduced information signal in, for example, an audio tape recorder, are well-known in the art. Such noise reduction systems are designed to increase the dynamic range of the signal that can be recorded and reproduced from a recording medium such as a magnetic tape. Such noise reduction systems generally incorporate an encoding process which compresses the level of the information signal and amplifies the high frequency components thereof prior to recording the signal on the recording medium. A decoding process is provided during the reproducing operation which is complementary to the compression and amplification which occur during the recording operation. In other words, the decoding process results in an expansion of the level of the reproduced information signal and an attenuation of the high frequency components thereof.
In one known noise reduction system, during the recording operation, the information signal is first transmitted through a pre-emphasis circuit which is provided with a large gain for pre-emphasizing the high frequency components of the signal supplied thereto. The pre-emphasized signal from the pre-emphasis circuit is then supplied to a compression circuit which compresses the pre-emphasized signal when the level of the information signal is less than a predetermined value and which transmits the signal with substantially unity gain for signal levels greater than the predetermined value. For example, the compression circuit may include a variable gain amplifier, such as a voltage controlled amplifier, which acts on the output from the pre-emphasis circuit. A control voltage for varying the gain of the variable gain amplifier is derived from a level detecting circuit and a limiter circuit in response to the output from the variable gain amplifier. When the level of the information signal is less than the predetermined value, the limiter circuit is inoperative so that the control signal from the level detecting circuit results in the variable gain amplifier imparting a compression characteristic to the information signal and when the level of the information signal is greater than the predetermined value, the limiter circuit grounds the control signal so that the variable gain amplifier transmits the information signal with substantially unity gain. Complementary de-emphasis and expansion circuitry are provided during the reproducing operation.
However, there are certain disadvantages with such noise reduction system. For example, in addition to the variable gain amplifier requiring adjustment, the limiter circuit must also be precisely adjusted for correct operation and such adjustment is often difficult. In addition, such noise reduction system exhibits a poor transient characteristic for changes in the level of the information signal. In particular, the compression circuit of the noise reduction system fails to adequately respond when the level of the information signal is abruptly increased, resulting in overshoots being created in the information signal. This occurs because the gain of the variable gain amplifier cannot instantaneously change with changes in the signal level with the result that the large gain normally applied to low level signals is applied to the high level signals for a short time when the level of the information signal abruptly rises from a low to a high level. This results in a so-called "noise breathing" phenomenon. This is particularly noticeable when the level of the information signal changes from a level below the aforementioned predetermined value to a level above the predetermined value whereby the gain of the variable gain amplifier cannot change instantaneously from its compression gain characteristic to its substantially unity gain characteristic. Such overshoots in the level of the information signal result in a distorted waveform being recorded and reproduced from the recording medium due to magnetic saturation of the recording tape.